New approaches in the combinatorial chemistry have resulted in the capability of producing millions of compounds in a short time. Analysis of each compound with respect to multiple parameters is proving to be a significant bottleneck as in e.g. M. A. Shoffner et al., Nucleic Acids Research, 1996, vol. 24, No. 2, pp. 375-9. The number of cells, the test reagent volumes, the throughput rate and the ease of use through automation are all important parameters which should be optimized in order to meet the stringent requirements for modem drug screening. Furthermore a small amount of precious reagent reduces both cost and waste, and increases the number of possible analyses. A candidate for this kind of analysis is a calorimeter. A calorimeter is a device which yields an electrical output signal but has an input or intermediate signal of the thermal type. Calorimetry, more than pH-metry, offers the advantage of generality: all chemical and physical processes are accompanied by changes in heat content, or enthalpy. In fact microcalorimeters can be used for the analysis of the activity of biological cells, chemical reactions in small volumes and other microanalytical applications.
The most frequently used commercially available calorimeters are the Thermometric 2277 Thermal Activity Monitor and the MicroCal MCS Isothermal Titration Calorimeter. They are both based on the use of two or more thermo-electric devices, so called thermopiles, having a common heat sink as reference. A thermopile is at least one thermocouple which is a temperature sensing element and which is connected to identical thermocouples in parallel thermally and in series electrically. Thermocouples do not measure the temperature itself, but rather the temperature difference between two junctions. An advantage of using thermocouples as temperature sensing elements is that there is no offset, i.e. when there is no temperature difference there is no voltage, which makes calibration superfluous. A thermocouple as illustrated in FIG. 2, i.e. a combination of two different (semi)conductive materials, converts a thermal difference between its two junctions into a voltage difference by means of the combined Seebeck coefficient S of its two structural thermoelectric materials. In fact a thermocouple comprises a first conductive material (14) and a second conductive material (13) with an insulating layer (15) inbetween. A thermocouple has a so-called hot junction (11), where said first material and said second material are short-circuited, and a so-called cold junction (15), where said first and said second material are separated one from another by means of said insulating layer. At said cold junction the electrical output signal, representing the temperature difference ΔT between said hot junction and said cold junction, can be measured.
The total generated voltage is the sum of the thermocouple voltages. For n (n being a positive whole number greater than zero) thermocouples, where each thermocouple is identical, it can be written that:Utp=n*S*ΔT.
The temperature difference ΔT is the product of the generated power difference between the two junction sites and the thermal resistance:ΔT=ΔPgen*RthThermopiles are preferred because they are self-generating, easy to integrate and because the temperature changes involved are low frequency signals.
The drawbacks of these state-of-the art devices are the following. These devices have at least two thermopiles and a common heat sink. The cold junctions of each thermopile are thermally coupled to the common heat sink which is at a known temperature. The hot junctions of each thermopile are thermally coupled to a substance under test. So in fact, one tries to perform a kind of absolute measurement by measuring the temperature difference between this substance under test and the heat sink at known temperature. By applying different substances under test to different thermopiles as e.g. for drug screening where the hot junctions of a first thermopile are coupled to reference cells and the hot junctions of a second thermopile are coupled to genetically engineered cells expressing a drug target. When the potential drug candidate is effective, it will activate the genetically engineered cells which results in a heat change. This heat change is determined indirectly by subtracting the measured signals of the first and the second thermopile, where the cold junctions of both thermopiles are coupled to a common heat sink at known temperature. This is a cumbersome approach which lacks accuracy and demands a space consuming design.